udp,pasta: Periodically scan for ports to automatically forward

pasta supports automatic port forwarding, where we look for listening
sockets in /proc/net (in both namespace and outside) and establish port
forwarding to match.

For TCP we do this scan both at initial startup, then periodically
thereafter.  For UDP however, we currently only scan at start.  So unlike
TCP we won't update forwarding to handle services that start after pasta
has begun.

There's no particular reason for that, other than that we didn't implement
it.  So, remove that difference, by scanning for new UDP forwards
periodically too.  The logic is basically identical to that for TCP, but it
needs some changes to handle the mildly different data structures in the
UDP case.

Link: https://bugs.passt.top/show_bug.cgi?id=45
Link: https://github.com/rootless-containers/rootlesskit/issues/383
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.1

@@ -496,8 +496,7 @@ Default is \fBauto\fR.
 .BR \-u ", " \-\-udp-ports " " \fIspec
 Configure UDP port forwarding to namespace. \fIspec\fR is as described for TCP
 above, and the list of ports is derived from listening sockets reported by
-\fI/proc/net/udp\fR and \fI/proc/net/udp6\fR, see \fBproc\fR(5),
+\fI/proc/net/udp\fR and \fI/proc/net/udp6\fR, see \fBproc\fR(5).
-when \fBpasta\fR starts (not periodically).
 
 Note: unless overridden, UDP ports with numbers corresponding to forwarded TCP
 port numbers are forwarded too, without, however, any port translation. 

udp.c

@@ -1186,6 +1186,66 @@ static void udp_timer_one(struct ctx *c, int v6, enum udp_act_type type,
 	}
 }
 
+/**
+ * udp_port_rebind() - Rebind ports to match forward maps
+ * @c:		Execution context
+ * @outbound:	True to remap outbound forwards, otherwise inbound
+ *
+ * Must be called in namespace context if @outbound is true.
+ */
+static void udp_port_rebind(struct ctx *c, bool outbound)
+{
+	const uint8_t *fmap
+		= outbound ? c->udp.fwd_out.f.map : c->udp.fwd_in.f.map;
+	const uint8_t *rmap
+		= outbound ? c->udp.fwd_in.f.map : c->udp.fwd_out.f.map;
+	struct udp_splice_port (*socks)[NUM_PORTS]
+		= outbound ? udp_splice_ns : udp_splice_init;
+	unsigned port;
+
+	for (port = 0; port < NUM_PORTS; port++) {
+		if (!bitmap_isset(fmap, port)) {
+			if (socks[V4][port].sock >= 0) {
+				close(socks[V4][port].sock);
+				socks[V4][port].sock = -1;
+			}
+
+			if (socks[V6][port].sock >= 0) {
+				close(socks[V6][port].sock);
+				socks[V6][port].sock = -1;
+			}
+
+			continue;
+		}
+
+		/* Don't loop back our own ports */
+		if (bitmap_isset(rmap, port))
+			continue;
+
+		if ((c->ifi4 && socks[V4][port].sock == -1) ||
+		    (c->ifi6 && socks[V6][port].sock == -1))
+			udp_sock_init(c, outbound, AF_UNSPEC, NULL, NULL, port);
+	}
+}
+
+/**
+ * udp_port_rebind_outbound() - Rebind ports in namespace
+ * @arg:	Execution context
+ *
+ * Called with NS_CALL()
+ *
+ * Return: 0
+ */
+static int udp_port_rebind_outbound(void *arg)
+{
+	struct ctx *c = (struct ctx *)arg;
+
+	ns_enter(c);
+	udp_port_rebind(c, true);
+
+	return 0;
+}
+
 /**
  * udp_timer() - Scan activity bitmaps for ports with associated timed events
  * @c:		Execution context
@@ -1197,6 +1257,20 @@ void udp_timer(struct ctx *c, const struct timespec *ts)
 	unsigned int i;
 	long *word, tmp;
 
+	if (c->mode == MODE_PASTA) {
+		if (c->udp.fwd_out.f.mode == FWD_AUTO) {
+			port_fwd_scan_udp(&c->udp.fwd_out.f, &c->udp.fwd_in.f,
+					  &c->tcp.fwd_out);
+			NS_CALL(udp_port_rebind_outbound, c);
+		}
+
+		if (c->udp.fwd_in.f.mode == FWD_AUTO) {
+			port_fwd_scan_udp(&c->udp.fwd_in.f, &c->udp.fwd_out.f,
+					  &c->tcp.fwd_in);
+			udp_port_rebind(c, false);
+		}
+	}
+
 	if (!c->ifi4)
 		v6 = 1;
 v6: